Technical Field
The present disclosure relates to water treatment processes using nanomaterials in membrane synthesis and surface modification. More particularly, the present disclosure relates to systems and methods for using a layer-by-layer (LbL) assembly of graphene oxide (GO) nanosheets via bonding techniques, such as covalent bonding and electrostatic interaction.
Description of Related Art
Dwindling water resources and increasing water consumption have forced researchers to consider new advanced water treatment technologies that can provide a safe water supply in a more efficient, environmentally sustainable manner. Nanofiltration (NF), reverse osmosis (RO), and forward osmosis (FO) membrane processes are among the most effective strategies to achieve high removal of both traditional and emerging contaminants from water. All these processes require the use of semi-permeable membranes, the market of which has been dominated for decades by thin film composite (TFC) polyamide membranes due to their salient advantages, such as good separation capability and wide pH tolerance. Despite their advantages, TFC membranes face technical limitations regarding, for example, chlorine resistance, fouling resistance, and energy efficiency. It is also a challenge to make TFC membranes with thinner, more hydrophilic and more porous support layers, which are crucial for high-performance membranes.
The recently emerging graphene-based nanomaterials have exhibited interesting properties, such as adsorption of metal and organic dyes, antimicrobial capability, and photocatalytic degradation of organic molecules. In particular, graphene oxide (GO) nanosheets offer an extraordinary potential for making functional nanocomposite materials with high chemical stability, strong hydrophilicity, and excellent antifouling properties. In recent years, nanomaterials have been extensively used in membrane synthesis and surface modification to improve membrane performance (e.g., flux, antibacterial property, fouling resistance, photocatalytic property) or to optimize the operation of membrane processes (e.g., energy consumption, maintenance requirement). Because the use of these nanomaterials often relies on expensive materials, costly facilities, and highly complex synthesis, it becomes very desirable to make high-performance water separation membranes using low-cost raw materials and facile yet scalable synthesis methods.
As a derivative of graphene, GO nanosheets can be mass-produced via chemical oxidization and ultrasonic exfoliation of graphite. Hence, GO nanosheets bear hydroxyl, carboxyl, and epoxide functional groups on the plane of carbon atoms and thus have a more polar, hydrophilic character. A GO nanosheet is single-atom-thick with lateral dimensions as high as tens of micrometers, making it highly stackable. Stacked GO nanosheets made via a simple solution filtration method can exhibit excellent mechanical strength in dry conditions.
The concept of using graphene-based nanomaterials to make water separation membranes was first examined using molecular simulations. Nanopores are “punched” through a super-strong graphene monolayer so that water can permeate through the single-atom-thick membrane while other substances are selectively rejected. By controlling pore sizes and functional groups on graphene, such a monolayer graphene membrane could be useful for desalination, with a water permeability of several magnitudes higher than that of current reverse osmosis (RO) membranes. An experimental study was recently reported to create such porous graphene membranes and test their selectivity for gas separation. Despite these simulation and experimental efforts, at present significant technical difficulties exist in making such monolayer graphene membrane for real-world water separation. For example, it is still impractical to prepare a large area of monolayer graphene, and it is extremely challenging to obtain high-density nanopores with controllable, relatively uniform sizes on a graphene sheet.
An alternative approach is to synthesize a water separation membrane with stacked GO nanosheets. The spacing between the neighboring GO nanosheets creates 2D nanochannels that may allow water to pass through while rejecting unwanted solutes. Water can flow at an extremely high speed in such planar graphene nanochannels. A recent experimental study has revealed unimpeded permeation of water vapor (at a rate 1010 times faster than helium) through a stacked GO membrane, a phenomenon that could be attributed to a nearly frictionless flow of a monolayer of water through 2D capillaries formed by closely spaced GO nanosheets. Although tested for gas/vapor separation only, stacked GO nanosheets hold great potential for making highly permeable water separation membranes to remove various types of contaminants.
Stacked GO membranes reported so far in the literature, however, are made simply via solution filtration. Hence, they are not suitable for water separation applications due to the lack of necessary bonding between stacked GO nanosheets. This is because GO nanosheets are extremely hydrophilic and thus these membranes tend to easily disperse in water. Even if some performance data could be collected through extremely careful handling of the membrane made with unbonded GO nanosheets, such a GO membrane unfortunately does not survive the cross-flow testing conditions, which are typical in real-world membrane operation. Therefore, these unbonded GO membranes should not be considered or used as water separation membranes.
After a GO membrane has been synthesized, the oxygen-containing functional groups on GO provide convenient sites for further functionalization to adjust various properties (e.g., charges, interlayer spacing, specific interactions with water contaminants) of GO nanosheets. For example, GO can be covalently functionalized by amine groups to modify charges, sulfonic groups to make ion/proton-exchange membranes, and polymers to enhance biocompatibility. GO can also be non-covalently bonded with various monomers, polymers, and even nanoparticles to adjust mechanical, thermal, and chemical properties. These exceptional properties of GO provide for flexibility to optimize not only membrane permeability by varying the size and morphology of the functional groups (thus adjusting GO interlayer spacing) but also membrane selectivity by adjusting charge, charge density, and specific interactions with water contaminants.
To date, however, synthesis of a water separation membrane by the proper bonding and optimization of stacked GO nanosheets has not been reported.